Enhanced cycling stability of ring-shaped phosphorus inside multi-walled carbon nanotubes as anodes for lithium-ion batteries

Abstract

A ring-shaped phosphorus allotrope has recently been synthesized inside multi-walled carbon nanotubes (r-P@MWCNTs). The electrochemical properties of the r-P@MWCNTs as anodes for lithium-ion batteries have been investigated. External phosphorus has been found to degrade rapidly during charging and discharging cycles. The internal ring-shaped phosphorus is well stabilized by the surrounding carbon nanotube walls to provide an extremely high cycling stability. The specific capacities of r-P@MWCNTs degrade in the first dozens of cycles and then tend to be stable. The insertion and extraction of lithium ions through MWCNTs to the encapsulated phosphorus are believed to occur through the defects and interlayer gaps of MWCNTs. The volume expansion of phosphorus during lithiation is eased along the axis of MWCNTs since the MWCNTs are not fully filled by phosphorus; this prevents the encapsulated phosphorus from escaping and further resulting in capacity degradation. A specific capacity of 444 mA h g⁻¹ based on the whole composites, containing about 16% phosphorus, is still maintained after 500 cycles at a current rate of 500 mA g⁻¹. The electrochemical performances of ring-shaped phosphorus inside MWCNTs have been demonstrated to be much higher than those of different phosphorus phases grown on the outside surfaces of MWCNTs under same conditions.